An image forming apparatus of electro-photographic system such as a laser printer is generally provided with a photosensitive drum on which an electrostatic latent image is formed. A developing cartridge for developing the electrostatic latent image into a toner image is detachably mounted to the image forming apparatus.
A cover is provided on a body of the apparatus so as to be opened/closed. By opening this cover, the developing cartridge can be mounted to and detached from the apparatus body.
The developing cartridge is provided with a developing roller for supplying toner to the photosensitive drum. This developing roller can be pressed against or separated from the photosensitive drum in a state where the developing cartridge has been mounted to the apparatus body.
As a structure for allowing the developing roller to be pressed against or separated from the photosensitive drum, a structure including a lever for pressing a developing unit which holds the developing roller, a guide member for operating the lever, and a guide cam for rendering this guide member to reciprocally and linearly move has been proposed (see JP-A-2002-6716, for example). When an image is to be formed, the guide member is moved in one direction following rotation of the guide cam, and the lever is separated from the developing unit, whereby the developing roller is pressed against the photosensitive drum. On the other hand, when an image is not formed, the guide member is moved in the other direction following the rotation of the guide cam, so that the lever presses the developing unit, whereby the developing roller is separated from the photosensitive drum.